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An electron spectroscopy study of ammonia adsorption on clean and oxidized aluminum
A253 clean surface of the 7 x 7 structure, and during the transformations of the 7 X 7 structure to the 5 X 1, ~/3 x ~/3 and 6 X 6 structures produced by deposition of gold up to a few monatomic layers. The reflection images showed the microtopographical aspects of the process and of the transformations to be in close correlation with the structural information given by the reflection diffraction patterns observed simultaneously.
Surface Science 97 (1980) 409-424 ©North-Holland Publishing Company
ADSORPTION OF BROMINE ON THE RECONSTRUCTED Au(100) SURFACE: LEED, THERMAL DESORPTION AND WORK FUNCTION MEASUREMENTS E. B E R T E L and F.P. N E T Z E R
lnstitut ffir Physikalische Chemie, Universita't[nnsbruck, A-6020 Innsbruck, Austria Received 6 February 1980; accepted for publication 3 April 1980 The chemisorption of bromine on Au(100) has been investigated using LEED, temperature programmed thermal desorption and work function measurements. The (5 × 20) surface structure of the clean Au(100) surface was found to reconstruct back to a (1 X I) surface upon low bromine exposures. A c(2 X 2) Br LEED structure was first observed at 0 ~ 0.4, which transforms easily into a "c(4 × 2)R45 °'' structure (0 = 0.5) within a narrow coverage range. At saturation a compressed bromine adsorbate layer is Ibrmed with 0 ~ 0.55 as derived from the LEED pattern. Up to 0 ~ 0.45, an essentially constant sticking probability of about 0.2 was found, which decre/~ses then sharply and is very low during the compression stage of the adsorbate layer. The work function increases linearly during bromine adsorption up to a value of , ~ = 800 mV at 0 ~ 0.5. A slight decrease from this value is observed during the beginning of the compression stage. Thermal desorption of bromine from Au(100) occurs in atomic form and reveals two main desorption features: A sharp peak around 800 K and a very broad feature starting to appear at surface coverages exceeding 0.25. Computer analysis of the desorption curves shows large variations of the desorption energy and the pre-exponential factor with surface coverage and a pronounced compensation effect. The results slightly favour an adsorbate layer model with bromine atoms in the bridge positions and strongly coverage dependent lateral interactions.
Surface Science 97 (1980) 425-447 © North-Holland Publishing Company
AN ELECTRON SPECTROSCOPY STUDY OF AMMONIA ADSORPTION ON CLEAN AND OXIDIZED ALUMINUM ? J.W. R O G E R S , Jr. *, C.T. C A M P B E L L :~, R.L. H A N C E § and J.M. WHITE
Department of Chemistry, University of Texas, Austin, Texas 78712, USA Received 21 January 1980 Photoelectron spectra for ammonia adsorbed in submonolayer and multilayer amounts on clean and oxidized aluminum have been measured and interpreted. Uptake at 128 K is dominated by weak molecular adsorption and saturates at submonolayer amounts whereas at 106 K multilayers can be formed. A tiny amount of dissociative adsorption is noted. Dissociation is promoted by exposure to Ar ÷ ion bombardment. At 128 K the initial sticking coefficient is 0.13 -+ 0.08. On oxidized A1, as compared to dean, the saturation amount of adsorbed NH 3 is
A254 larger and it is more tightly held. In both cases the work function change is consistent with a model in which the ammonia adsorbs with the nitrogen atom toward the surface. On clean A1, there is good evidence that NH 3 tends to be physisorbed with the major bonding arising between the dipole of NH3, and the image dipole induced in the metal. This is supported by the resemblence of the separation of the valence orbitals to the separation in the gas phase, as well as their shift to higher binding energy with increasing coverage. On oxidized AI the bonding is similar but the bond strength is greater. There is no evidence for significant NH,~ formation on either clean or oxidized AI. The Lewis acidity of the substrate increases with the extent of surface oxidation. The bonding of NH 3 to oxidized AI is very similar to the intermolecular bonding in an ammonia molecular crystal.
Surface Science 97 (1980) 448 456 f©North-tlolland Publishing Company I N F L U E N C E O F A D S O R P T I O N UPON S O U N D R E F L E C T I O N Peter K R I ~ A N and Ivan K U g ~ E R
Department of Physics, University of Ljubljana, P.O. Box 543, Ljubl/ana, Yt~goslavia Received 13 February 1980; accepted for publication 9 April 1980 The reflectance of sound incident from a gas upon an adsorbing surface depends upon frequency and involves three parameters: the sticking coefficient and two relaxation times, one related to the desorption rate, and the other depending upon thermal properties of the system. An extension of the frequency response technique to acoustic frequencies is suggested thereby. Its applicability is limited by the requirement that the reciprocal relaxation times be well below the collision frequency in the gas. Multilayer adsorption upon metal surfaces appears as a favourable case.
Surface Science 97 (1980) 457-477 © North-Holland Publishing Company
FACETING OF Cu(210) AND Ni(210) BY ACTIVATED NITROGEN R . E . K I R B Y *, C.S. M c K E E and L.V. R E N N Y **
School of Chemistry, University of BradJbrd, Bradford BD 71DP, West Yorkshire, UK Received 12 July 1979; accepted for publication 11 March 1980 The interactions of activated nitrogen (AN) with Cu(210) and Ni(210) and of 02 with Ni(210) have been studied using LEED, RHEED and AES. Exposure to AN at 290 K produces (100) facets with a c(11,,/2 ×.,/2)R45°-N overlayer; on warming (520 K) in vacuum, (110) facets with a p(2 X 3)-N overlayer appear. Heating to 600 K causes desorption of N2 and regeneration of the (210) surface. With 02 on Ni(210) complicated facet patterns are observed. Facet orientations have been identified by a simple method involving rotation of the crystal. The results are discussed in relation to the stabilities of various Cu surfaces, particularly (410) and(530), in the presence of AN and 02.
Surface Science 97 (1980) 409-424 ©North-Holland Publishing Company
ADSORPTION OF BROMINE ON THE RECONSTRUCTED Au(100) SURFACE: LEED, THERMAL DESORPTION AND WORK FUNCTION MEASUREMENTS E. B E R T E L and F.P. N E T Z E R
lnstitut ffir Physikalische Chemie, Universita't[nnsbruck, A-6020 Innsbruck, Austria Received 6 February 1980; accepted for publication 3 April 1980 The chemisorption of bromine on Au(100) has been investigated using LEED, temperature programmed thermal desorption and work function measurements. The (5 × 20) surface structure of the clean Au(100) surface was found to reconstruct back to a (1 X I) surface upon low bromine exposures. A c(2 X 2) Br LEED structure was first observed at 0 ~ 0.4, which transforms easily into a "c(4 × 2)R45 °'' structure (0 = 0.5) within a narrow coverage range. At saturation a compressed bromine adsorbate layer is Ibrmed with 0 ~ 0.55 as derived from the LEED pattern. Up to 0 ~ 0.45, an essentially constant sticking probability of about 0.2 was found, which decre/~ses then sharply and is very low during the compression stage of the adsorbate layer. The work function increases linearly during bromine adsorption up to a value of , ~ = 800 mV at 0 ~ 0.5. A slight decrease from this value is observed during the beginning of the compression stage. Thermal desorption of bromine from Au(100) occurs in atomic form and reveals two main desorption features: A sharp peak around 800 K and a very broad feature starting to appear at surface coverages exceeding 0.25. Computer analysis of the desorption curves shows large variations of the desorption energy and the pre-exponential factor with surface coverage and a pronounced compensation effect. The results slightly favour an adsorbate layer model with bromine atoms in the bridge positions and strongly coverage dependent lateral interactions.
Surface Science 97 (1980) 425-447 © North-Holland Publishing Company
AN ELECTRON SPECTROSCOPY STUDY OF AMMONIA ADSORPTION ON CLEAN AND OXIDIZED ALUMINUM ? J.W. R O G E R S , Jr. *, C.T. C A M P B E L L :~, R.L. H A N C E § and J.M. WHITE
Department of Chemistry, University of Texas, Austin, Texas 78712, USA Received 21 January 1980 Photoelectron spectra for ammonia adsorbed in submonolayer and multilayer amounts on clean and oxidized aluminum have been measured and interpreted. Uptake at 128 K is dominated by weak molecular adsorption and saturates at submonolayer amounts whereas at 106 K multilayers can be formed. A tiny amount of dissociative adsorption is noted. Dissociation is promoted by exposure to Ar ÷ ion bombardment. At 128 K the initial sticking coefficient is 0.13 -+ 0.08. On oxidized A1, as compared to dean, the saturation amount of adsorbed NH 3 is
A254 larger and it is more tightly held. In both cases the work function change is consistent with a model in which the ammonia adsorbs with the nitrogen atom toward the surface. On clean A1, there is good evidence that NH 3 tends to be physisorbed with the major bonding arising between the dipole of NH3, and the image dipole induced in the metal. This is supported by the resemblence of the separation of the valence orbitals to the separation in the gas phase, as well as their shift to higher binding energy with increasing coverage. On oxidized AI the bonding is similar but the bond strength is greater. There is no evidence for significant NH,~ formation on either clean or oxidized AI. The Lewis acidity of the substrate increases with the extent of surface oxidation. The bonding of NH 3 to oxidized AI is very similar to the intermolecular bonding in an ammonia molecular crystal.
Surface Science 97 (1980) 448 456 f©North-tlolland Publishing Company I N F L U E N C E O F A D S O R P T I O N UPON S O U N D R E F L E C T I O N Peter K R I ~ A N and Ivan K U g ~ E R
Department of Physics, University of Ljubljana, P.O. Box 543, Ljubl/ana, Yt~goslavia Received 13 February 1980; accepted for publication 9 April 1980 The reflectance of sound incident from a gas upon an adsorbing surface depends upon frequency and involves three parameters: the sticking coefficient and two relaxation times, one related to the desorption rate, and the other depending upon thermal properties of the system. An extension of the frequency response technique to acoustic frequencies is suggested thereby. Its applicability is limited by the requirement that the reciprocal relaxation times be well below the collision frequency in the gas. Multilayer adsorption upon metal surfaces appears as a favourable case.
Surface Science 97 (1980) 457-477 © North-Holland Publishing Company
FACETING OF Cu(210) AND Ni(210) BY ACTIVATED NITROGEN R . E . K I R B Y *, C.S. M c K E E and L.V. R E N N Y **
School of Chemistry, University of BradJbrd, Bradford BD 71DP, West Yorkshire, UK Received 12 July 1979; accepted for publication 11 March 1980 The interactions of activated nitrogen (AN) with Cu(210) and Ni(210) and of 02 with Ni(210) have been studied using LEED, RHEED and AES. Exposure to AN at 290 K produces (100) facets with a c(11,,/2 ×.,/2)R45°-N overlayer; on warming (520 K) in vacuum, (110) facets with a p(2 X 3)-N overlayer appear. Heating to 600 K causes desorption of N2 and regeneration of the (210) surface. With 02 on Ni(210) complicated facet patterns are observed. Facet orientations have been identified by a simple method involving rotation of the crystal. The results are discussed in relation to the stabilities of various Cu surfaces, particularly (410) and(530), in the presence of AN and 02.